Base oils are commonly used for the production of lubricants, such as lubricating oils for automotives, industrial lubricants and lubricating greases. They are also used as process oils, white oils, metal working oils and heat transfer fluids. Finished lubricants consist of two general components, lubricating base oil and additives. Lubricating base oil is the major constituent in these finished lubricants and contributes significantly to the properties of the finished lubricant. In general, a few lubricating base oils are used to manufacture a wide variety of finished lubricants by varying the mixtures of individual lubricating base oils and individual additives.
According to the American Petroleum Institute (API) classifications, base oils are categorized in five groups based on their saturated hydrocarbon content, sulfur level, and viscosity index (Table 1). Lube base oils are typically produced in large scale from non-renewable petroleum sources. Group I, II, and III base stocks are all derived from crude oil via extensive processing, such as solvent extraction, solvent or catalytic dewaxing, and hydroisomerization. Group III base oils can also be produced from synthetic hydrocarbon liquids obtained from natural gas, coal or other fossil resources. Group IV base stocks, the polyalphaolefins (PAO), are produced by oligomerization of alpha olefins, such as 1-decene. Group V base oils include everything that does not belong to Groups I-IV, such as naphthenics, polyalkylene glycols (PAG), and esters.
TABLE 1APIclassificationGroup IGroup IIGroup IIIGroup IVGroup V% Saturates<90≧90≧90Poly-All others% S>0.03≦0.03≦0.03alphaolefinsnotViscosity80-12080-120≧120(PAO)belongingIndex (VI)to groupI-IV
The automotive industry has been using lubricants and thus base oils with improved technical properties for a long time. Increasingly, the specifications for finished lubricants require products with excellent low temperature properties, high oxidation stability and low volatility. Generally lubricating base oils are base oils having kinematic viscosity of about 3 cSt or greater at 100° C. (Kv100); pour point (PP) of about −12° C. or less; and viscosity index (VI) about 90 or greater. In general, high performance lubricating base oils should have a Noack volatility no greater than current conventional Group I or Group II light neutral oils. Currently, only a small fraction of the base oils manufactured today are able to meet these demanding specifications.
For environmental, economical, and regulatory reasons, it is of interest to produce fuels, chemicals, and lube oils from renewable sources of biological origin. So far only esters of renewable and biological origin have been used in applications such as refrigeration compressor lubricants, bio-hydraulic oils and metal working oils. In automotive and industrial lubricants, esters from biological sources are used in very small fractions as additives due to technical problems as well as their high prices. For example, ester base oils can hydrolyze readily producing acids, which in turn cause corrosion on lubricating systems.
In contrast, base oils consisting of hydrocarbons from biological sources do not have those technical problems associated with esters from same sources. Most common biological sources for hydrocarbons are natural oils, which can be derived from plant sources such as canola oil, castor oil, sunflower seed oil, rapeseed oil, peanut oil, soy bean oil, and tall oil, or derived from animal fats. The basic structural unit of natural oils and fats is a triglyceride, which is an ester of glycerol with three fatty acid molecules having the structure below:
wherein R1, R2, and R3 represent C4-C30 hydrocarbon chains. Fatty acids are carboxylic acids containing long linear hydrocarbon chains. Lengths of the hydrocarbon chains most commonly are 18 carbons (C18). C18 fatty acids are typically bonded to the middle hydroxyl group of glycerol. Typical carbon numbers of the fatty acids linked to the two other hydroxyl groups are even numbers, being between C14 and C22. Fatty acid composition of biological origin may vary considerably among feed-stocks from different sources. While several double bonds may be present in fatty acids, they are non-conjugated (with at least one —CH2— unit between the double bonds). With respect to configuration, the double bonds of natural fatty acids are mostly of cis form. As the number of the double bonds increase, they are generally located at the free end of the chain. Lengths of hydrocarbon chains and numbers of double bonds depend on the various plant or animal fats or waxes serving as the source of the fatty acid. Animal fats typically contain more saturated fatty acids than unsaturated fatty acids. Fatty acids of fish oil contain high amounts of double bonds, and the average length of the hydrocarbon chains is higher compared to fatty acids of plant oils and animal fats.
Prior to processing, starting materials of biological origin are commonly pretreated with any suitable known methods such as thermally, mechanically for instance by means of shear forces, chemically for instance with acids or bases, or physically with radiation, distillation, cooling, or filtering. The purpose of said chemical and physical pretreatments is to remove impurities interfering with the process or poisoning the catalysts, and reduce unwanted side reactions.
In a hydrolysis treatment, oils and fats react with water yielding free fatty acids and glycerol as the product. Three main processes for the industrial production of fatty acids are known: vapor splitting of triglycerides under high pressure, basic hydrolysis, and enzymatic hydrolysis. In the vapor splitting process, the hydrolysis of triglycerides using steam is carried out at temperatures between 100 and 300° C., under a pressure of 1-10 MPa, preferable conditions being from 250 to 260° C. and from 4 to 5.5 MPa. Metal oxides like zinc oxide may be added as the catalyst to accelerate the reaction.
The unsaturated fatty acids obtained from hydrolysis of natural oils can be dimerized to form dimers of unsaturated fatty acids. A variety of dimerization processes have been described. For example, in Kirk-Othmer: Encyclopedia of Chemical Technology, 3rd Ed., vol. 7, Dimer acids, p. 768, a method is presented for producing dimeric acids from unsaturated carboxylic acids with a radical reaction using a cationic catalyst, the reaction temperature being 230° C. In addition to acyclic unsaturated dimeric acid as the main product, mono- and bi-cyclic dimers are also formed. In Koster R. M. et al., Journal of Molecular Catalysis A: Chemical 134 (1998) 159-169, oligomerization of carboxylic acids, carboxylic acid methyl esters, and synthetic alcohols and olefins is described, yielding corresponding dimers.
The oxygen atoms in carboxylic acids can be removed in the form of CO (decarbonylation), CO2 (decarboxylation), or H2O (deoxygenation). Processes wherein the oxygen of a carboxylic acid or ester is removed are known. Decarboxylation of fatty acids removes CO2 and results in hydrocarbons with one carbon atom less than the original molecule. The feasibility of decarboxylation varies greatly with the type of carboxylic acid used as the starting material. Activated carboxylic acids containing electron-withdrawing groups in the position alpha or beta with respect to the carboxylic group lose carbon dioxide readily at slightly elevated temperatures. In this case, the RC—COOH bond is weakened by the electron-withdrawing group on the carbon chain. With other types of carboxylic acids, the RC—COOH bond is strong and cleavage of carbon dioxide is difficult. A suitable catalyst is required for this reaction. For example, in Maier, W. F. et al., Chemische Berichte (1982), 115(2), 808-812, hydrocarbons are produced from carboxylic acids using heterogeneous Ni/Al2O3 and Pd/SiO2 catalysts at 180° C. under hydrogen atmosphere. Further examples of decarboxylation and hydrogenation of oxygen containing compounds are disclosed in Laurent, E., Delmon, B.: Applied Catalysis, A: General (1994), 109(1), 77-96, and 97-115, wherein pyrolysis oils derived from biomass were subjected to hydrogenation using sulfided CoMo/γ-Al2O3 and NiMo/γ-Al2O3 catalysts at 260-300° C., under a hydrogen pressure of 7 MPa.
In published U.S. Publication No. 2007/0131579, processes for converting unsaturated carboxylic acids to saturated hydrocarbons are described. The processes employ steps of: (a) oligomerization of unsaturated fatty acids forming dimer acids; (b) pre-hydrogenation to remove the C═C double bond(s); (c) de-oxygenation of the dimer acids in the form of decarboxylation and/or decarbonylation; and (d) optional hydrofinishing to remove double bonds and aromatics. Once the dimer acids are formed, tedious three steps are required in these disclosed processes to generate saturated hydrocarbons. Furthermore, this patent publication discloses a preferred product composition containing 20-90% naphthenes.
JP 76031241B discloses insulating oils formed by dimerization/trimerization of unsaturated fatty acids followed by hydrogenation. Oxygen atoms are removed in the form of water via hydrogenation, which requires two steps of hydrogenation to achieve.
With recent developments in biodiesel production, unsaturated fatty acids and their esters are increasingly available. Therefore it is desirable to take advantage of the renewable feed-stocks, thus saving non-renewable petroleum raw materials. Despite of the above teaching in the art, there is an need for an alternative and simpler process for producing saturated hydrocarbons from starting materials of biological origin, and to avoid the problems associated with the solutions disclosed in the prior art.